Atmos. Chem. Phys., 11, 7533–7545, 2011 www.atmos-chem-phys.net/11/7533/2011/ Atmospheric doi:10.5194/acp-11-7533-2011 Chemistry © Author(s) 2011. CC Attribution 3.0 License. and Physics Projections of UV radiation changes in the 21st century: impact of ozone recovery and cloud effects A. F. Bais1, K. Tourpali1, A. Kazantzidis2, H. Akiyoshi3, S. Bekki4, P. Braesicke5, M. P. Chipperfield6, M. Dameris7, V. Eyring7, H. Garny7, D. Iachetti8, P. Jockel¨ 7, A. Kubin9, U. Langematz9, E. Mancini8, M. Michou10, O. Morgenstern11, T. Nakamura3, P. A. Newman12, G. Pitari8, D. A. Plummer13, E. Rozanov14,15, T. G. Shepherd16, K. Shibata17, W. Tian6, and Y. Yamashita3 1Aristotle University of Thessaloniki, Department of Physics, Thessaloniki, Greece 2University of Patras, Department of Physics, Patras, Greece 3National Institute for Environmental Studies, Tsukuba, Japan 4Service d’Aeronomie, Institut Pierre-Simone Laplace, Paris, France 5University of Cambridge, Department of Chemistry, Cambridge, UK 6Institute for Climate and Atmospheric Science, University of Leeds, Leeds, UK 7Deutsches Zentrum fur¨ Luft- und Raumfahrt, Institut fur¨ Physik der Atmosphare,¨ Oberpfaffenhofen, Germany 8Universita` L’Aquila, Dipartimento di Fisica, L’Aquila, Italy 9Freie Universitat¨ Berlin, Institut fur¨ Meteorologie, Berlin, Germany 10GAME/CNRM (Met´ eo-France,´ CNRS), Toulouse, France 11National Institute of Water and Atmospheric Research, Lauder, New Zealand 12NASA Goddard Space Flight Center, Greenbelt, MD, USA 13Environment Canada, Victoria, BC, Canada 14Physical-Meteorological Observatory Davos/World Rad. Center, Davos, Switzerland 15Institute for Atmospheric and Climate Science ETH, Zurich, Switzerland 16Department of Physics, University of Toronto, Toronto, Canada 17Meteorological Research Institute, Tsukuba, Japan Received: 7 March 2011 – Published in Atmos. Chem. Phys. Discuss.: 6 April 2011 Revised: 19 July 2011 – Accepted: 20 July 2011 – Published: 1 August 2011 Abstract. Monthly averaged surface erythemal solar irradi- the spread in the CCM projections. The calculations suggest ance (UV-Ery) for local noon from 1960 to 2100 has been that relative to 1980, annually mean UV-Ery in the 2090s will derived using radiative transfer calculations and projections be on average ∼12 % lower at high latitudes in both hemi- of ozone, temperature and cloud change from 14 chemistry spheres, ∼3 % lower at mid latitudes, and marginally higher climate models (CCM), as part of the CCMVal-2 activity of (∼1 %) in the tropics. The largest reduction (∼16 %) is pro- SPARC. Our calculations show the influence of ozone deple- jected for Antarctica in October. Cloud effects are respon- tion and recovery on erythemal irradiance. In addition, we sible for 2–3 % of the reduction in UV-Ery at high latitudes, investigate UV-Ery changes caused by climate change due but they slightly moderate it at mid-latitudes (∼1 %). The to increasing greenhouse gas concentrations. The latter in- year of return of erythemal irradiance to values of certain clude effects of both stratospheric ozone and cloud changes. milestones (1965 and 1980) depends largely on the return of The derived estimates provide a global picture of the likely column ozone to the corresponding levels and is associated changes in erythemal irradiance during the 21st century. Un- with large uncertainties mainly due to the spread of the model certainties arise from the assumed scenarios, different param- projections. The inclusion of cloud effects in the calculations eterizations – particularly of cloud effects on UV-Ery – and has only a small effect of the return years. At mid and high latitudes, changes in clouds and stratospheric ozone trans- port by global circulation changes due to greenhouse gases Correspondence to: A. F. Bais will sustain the erythemal irradiance at levels below those ([email protected]) in 1965, despite the removal of ozone depleting substances. Published by Copernicus Publications on behalf of the European Geosciences Union. 7534 A. F. Bais et al.: Projections of UV radiation changes in the 21st century At northern high latitudes (60◦–90◦), the projected decreases 2 Data and methodology in cloud transmittance towards the end of the 21st century will reduce the yearly average surface erythemal irradiance 2.1 Simulations of erythemal irradiance by ∼5 % with respect to the 1960s. The spectral solar irradiance (flux on a horizontal surface) from 290 to 400 nm received at Earth’s surface is calculated 1 Introduction for the entire globe with a radiative transfer (RT) model (li- bRadtran) (Mayer and Kylling, 2005) based on input data Stratospheric ozone is expected to recover during the com- derived from different CCMs, covering the period from 1960 ing decades in response to the reduction in ozone depleting to 2100. In this study we used the REF-B2 and SCN-B2d substances that have been regulated by the Montreal Protocol simulations conducted in the framework of the CCMVal- and its amendments. However, increases in greenhouse gas 2 activity of SPARC (Eyring et al., 2008; Morgenstern et (GHG) concentrations are expected to interact with ozone al., 2010). In these simulations a prescribed scenario for and alter its spatial distribution and its exchange between the greenhouse gases (SRES A1b) (IPCC, 2001) and the A1 ad- stratosphere and the troposphere. These changes in ozone justed halogen scenario (WMO, 2007) were used. Compared will affect the ultraviolet radiation at the surface. Predictions to the previous model runs under CCMVal-1 (Eyring et al., of future UV radiation levels are important for policy plan- 2006), most models underwent further development targeting ning, as changes in UV radiation may have both adverse and the weaknesses identified in CCMVal-1, providing in gen- beneficial effects on humans and ecosystems (UNEP, 2006). eral more consistent ozone projections (Austin et al., 2010; Changes in UV radiation in the future will depend on changes Morgenstern et al., 2010). The CCMs used in this study are in various atmospheric factors, the most important being to- shown in Table 1 and further details are given in (Morgen- tal ozone, clouds, aerosols, and, at specific locations, surface stern et al., 2010). We also used results from a simulation reflectivity (or albedo). Other factors, such as gaseous pollu- of EMAC-FUB model which is a modified version of EMAC tants in the troposphere and temperature in the stratosphere, with improved representation of polar stratospheric clouds may also play a role (WMO, 2007). Without accounting but of lower vertical resolution (39 levels compared to 90 of for the effects of changes in these parameters, Tourpali et EMAC). al. (2009) estimated the changes in noontime erythemal irra- Simulations of global fields for total ozone and vertical diance based on simulations of total ozone columns and ver- profiles of ozone and air temperature from 14 models have tical profiles of ozone and temperature from 11 Chemistry- been used, while 5 models provided also the total (short- Climate Models (CCMs) incorporating stratospheric ozone wave) solar irradiance at the surface under cloud-free and recovery (Eyring et al., 2007). They estimated decreases in all-sky conditions. For some models projections were avail- erythemal irradiance of 5–15 % over mid-latitudes between able from multiple runs. For these models (including 3 of 2000 and 2100, and twice as much at southern high latitudes. the models that provided solar irradiance simulations) the en- These decreases were primarily due to the effects of ozone semble mean of the input parameters was calculated and used recovery. Hegglin and Shepherd (2009) calculated changes in the radiation calculations. As different models provided in the UV Index under clear skies based on ozone fields de- simulations on different grids, all output data were reduced ◦ ◦ rived from a CCM and an analytical formulation which was to a common grid of 5 ×5 . used to estimate the UV Index from ozone and solar zenith In addition to the CCM-derived projections, more realistic angle (Madronich, 2007). By examining the changes be- and spatially variable climatological conditions for the opti- tween 1965 and 2095, they were able to isolate the effects cal depth of aerosols (Kinne et al., 2006) and surface reflec- of climate change from those of ozone depletion and recov- tivity (Herman et al., 2001), compared to those of Tourpali ery from ozone depleting substances. Over the 1965–2095 et al. (2009), were used in the radiative transfer calculations. period they reported decreases of 9 % in the UV Index in As both parameters vary spatially and seasonally, their in- northern high latitudes, increases by 4 % in the tropics, and clusion in the RT calculations result in longitudinal and sea- large increases of up to 20 % in southern high latitudes in late sonal variations of the calculated irradiance fields. These spring and early summer. fields were assumed constant throughout the 21st century, In this study we update the results of (Tourpali et al., 2009) although in reality they would likely change as a result of using new CCM simulations and parameterizations for the changes in climate. No information on these parameters, us- effects of clouds. We discuss the changes in solar erythemal able directly in RT calculations, are included in the available irradiance during the 21st century calculated with a radiative CCM runs. It should be noted that these CCMs did not in- transfer model using projected ozone, temperature and cloud clude realistic representations of tropospheric chemistry, so fields from the most recent CCM simulations carried out for they do not predict changes in tropospheric ozone which also CCMVal-2 (SPARC CCMVal, 2010) and the “Scientific As- would be likely to affect surface UV radiation levels in the sessment of Ozone Depletion: 2010” (WMO, 2011). future. All CCM output parameters were used as monthly av- erages in the RT model calculations to simulate the spectral Atmos. Chem. Phys., 11, 7533–7545, 2011 www.atmos-chem-phys.net/11/7533/2011/ A. F. Bais et al.: Projections of UV radiation changes in the 21st century 7535 Table 1.